Hitherto, there have been known various non-contacting torque sensors of the type including a torque transmission shaft having formed on its surface a magnetically anisotropic portion comprising machined grooves and/or an amorphous thin magnetic belt, and detection coils disposed around the torque transmission shaft for detecting changes in the magnetic permeability of the magnetically anisotropic portion during torque transmission.
Typically, the torque sensor shaft to which a torque is applied has a pair of magnetically anisotropic portions formed by machining on its surface in the form of grooves or the like. The pair of magnetically anisotropic portions are inclined relative to the axis of the torque sensor shaft so that they are opposite to each other in the direction of magnetic anisotropy and so that their respective angles of inclination are equal to each other in absolute value (which angle of inclination is hereinafter referred to as "inclination angle of magnetic anisotropy"). An excitation coil or coils and detection coils are disposed around each magnetically anisotropic portion.
The inclination angle of magnetic anisotropy is defined to be ".+-. about 45 degree" in most prior publications, for example, in the specification of Japanese Patent No. 169326 and Japanese Patent Application Laid-Open Publication No. 62-185036. In some publications, for example, Japanese Patent Application Laid-Open Publication No. 63-252487, the angle is defined to be "0 to 90 degree", but no particular mention is given as to the ground therefor; as such, this definition is not particularly different in sense from ".+-.45 degree". Insofar as known torque sensors of this type are concerned, therefore, it may be understood that ".+-.45 degree" has been generally accepted as the inclination angle of magnetic anisotropy with respect to the magnetically anisotropic portions.
Usually, when a torque is applied to a torque sensor shaft, principal tensile and compressive stresses act in directions of .+-.45 degree relative to the axis of the shaft. Therefore, it has been considered that in torque sensors wherein the inclination angle of magnetic anisotropy with respect to the magnetically anisotropic portions is set at .+-.45 degrees as defined in known publications, directions in which principal stresses act when a torque is applied would coincide with the directions of magnetic anisotropy of the magnetically anisotropic portions so that stresses acting in the directions of magnetic anisotropy would be maximal, whereby most satisfactory detection sensitivity could be obtained.
Generally, however, the magnetic strength of a magneto-strictive material having an Ni content of 1 to 20% which is used for torque sensor shafts of this sort is less strong than its mechanical strength.
The term "magnetic strength" herein refers to a saturation characteristic with respect to stress-magnetism effect (inverse magneto-striction effect). More specifically, in a grooved type torque sensor as described in the specification of Japanese Patent No. 169326, for example, stress concentration occurs in groove bottom portions when a torque is applied, and hysteresis begins to increase with respect to the characteristics of the sensor from a point at which the magnitude of the stress concentration exceeds the strength of weakest crystals (about 10 to 20 kg f/mm.sup.2) present in a peripheral area of the groove bottom (including those present depthwise thereof). In this case, the magnitude of the stress by which the hysteresis of the sensor characteristics begins to increase is called magnetic strength. Generally, any magneto-strictive material used for torque sensor shafts is such that when the torque applied has exceeded the magnetic strength of the material, the torque is mechanically still within the elastic limits of the material. In this sense, the magnetic strength of the magneto-strictive material is lower than the mechanical strength thereof.
When torque is applied so that the torque load increases gradually from zero, the hysteresis characteristics and linearity of the sensor begins to become deteriorated under a considerably small stress relative to the mechanical strength of the sensor material and, in turn, a very large hysteresis results until what may be termed a breakdown of the magnetic characteristics has occurred. In the condition in which the magnetic strength is weaker than the mechanical strength in this way, attempts to provide a torque detecting region newly on various conventional shafts would require that almost all of the shafts be made larger in thickness than their existing condition. This is very disadvantageous from an economical point of view.